public class Example4_13 {
 public static void main(String args[] ) {
  Circle circle =new Circle();
  circle.setRadius(196.87);
  Tixing lader=new Tixing(3,21,9);
  Student zhang =new Student();
  System.out.println("zhang计算圆的面积：");
  double result = zhang.computerArea(circle);
  System.out.println(result);
  System.out.println("zhang计算梯形的面积:");
  result = zhang.computerArea(lader);
  System.out.println(result);
}
}

